A -90-dB THD rail-to-rail input opamp using a new local charge pump in CMOS

This paper describes the principle and design of a CMOS rail-to-rail input operational amplifier with THD performance of -90 dB which is suited for high-quality audio systems. A new output stage has been used featuring an output swing that extends to either supply rail and is capable of driving a low ohmic load (32 Omega), The opamp, which is realized in a 0.5-mu m 3.3-V digital CMOS process, uses a standard two-stage Miller configuration. The rad-to-rad input functionality is achieved with a new area-efficient on-chip charge pump which provides the local supply voltage far the input differential pair. THD levels below -90 dB have not yet been shown with existing rail-to-rail techniques. This rail-to-rail input configuration also behaves independently of the common mode level with respect to transconductance and slewing characteristics.